The Anatomy of the Axillary Nerve

The axillary nerve, which is also called the circumflex nerve, emerges from the posterior cord of a network of nerves called the brachial plexus right at the level of the armpit. It's a major peripheral nerve of the arm, carrying fibers from the fifth and sixth cervical vertebrae (C5 and C6), which are in near the base of the neck. The axillary nerve's primary purpose is to supply nerve function to the shoulder joint and three muscles in the arm, but it also innervates some skin in that region, as well.

Anatomy

With the exception of the cranial nerves, all the nerves of your body branch off from the spinal cord, emerge from between vertebrae, and then continue to branch off as they travel to various muscles and other structures throughout your body.

The axillary nerve gets its name from the axilla, which is the medical name for the armpit. You have two, one on each side (as you do with most nerves.) However, they're typically referred to as a single nerve, or as the left or right axillary nerve when the side needs to be specified.

After leaving the spinal column, the axillary nerve runs behind the axillary artery and continues down to the lower edge of the subscapularis muscle on the shoulder blade. It winds backward and travels down the arm along the posterior humeral circumflex artery.

The nerve then passes through an area called the quadrangular space before it further divides into its terminal branches, which are:

• The anterior (or "upper") branch, which winds around the neck of the humerus (funny bone) and goes beneath the deltoid muscle (the triangular muscle of the shoulder and upper arm). It then connects to the forward edge of the deltoid. It also gives off a few small branches that are cutaneous, which means that they serve the skin in that area.
• The posterior (or "lower") branch, which innervates the teres minor muscles and the lower part of the deltoid; it enters the deep fascia and becomes the superior lateral cutaneous nerve. It then wraps around the lower edge of the deltoid and connects to the skin over the lower two-thirds of the muscle, and covers the long head of the triceps brachii.
• The articular branch, which arises from the trunk of the axillary nerve and enters the glenohumeral joint, which is in the shoulder, below the subscapularis muscle.

The quadrangular space is a small area of your shoulder blade just above the armpit where there's a gap in the muscles. This gap provides space for nerves and blood vessels to pass through to the arm.

Anatomical Variations

It's important for surgeons to know about possible variations in the course of a nerve so they can avoid damage to them during procedures.

In a 2016 case report, healthcare providers noted an incidence of the axillary nerve branching directly off of the upper trunk of the brachial plexus rather than the posterior cord. In this case, it innervated the subscapularis muscle and latissimus dorsi in addition to the deltoid and teres minor muscles. It also had a communicating branch to the posterior cord.

A 2019 case report documented multiple abnormalities in the course of the axillary nerve in a woman with pain and severely limited motion in her shoulder. During reverse total shoulder arthroplasty, the surgeon discovered that the axillary nerve ran beside the coracoid process instead of beneath it, and it stayed close to the subscapularis muscle instead of traveling through the quadrangular space.

The case report noted earlier reports of axillary nerves not running through the quadrangular space, but in those cases, it either pierced the subscapularis muscle or split into branches before reaching the quadrangular space, but this woman's nerve did neither of these things.

Function

The axillary nerve functions as both a motor nerve (dealing with movement) and a sensory nerve (dealing with sensation, such as touch or temperature).

Motor Function

As a motor nerve, the axillary nerve innervates three muscles in the arm. These are the:

• Deltoid, which allows you to flex the shoulder joint and rotate your shoulder inward
• Long head of the triceps, down the back of the outer arm, which allows you to straighten your arm as well as pull the supper arm toward your body or extend it backward. This muscle can also be innervated by the radial nerve.
• Teres minor, one of the rotator cuff muscles, which starts at the outside of the shoulder and runs diagonally along much of the bottom edge of the shoulder blade and works with other muscles to allow for the external rotation of your shoulder joint

Sensory Function

In its sensory role, the axillary nerve carries information to the brain from the:

• Glenohumeral joint (the ball-and-socket joint in the shoulder)
• Skin covering the lower two-thirds of the deltoid muscle, via the superior lateral cutaneous branch

Associated Conditions

Problems with the axillary nerve can be caused by injuries anywhere along its route through the arm and shoulder, as well as by disease. Common injuries to the region include:

• Dislocations of the shoulder joint, which can cause axillary nerve palsy
• Fracture of the surgical neck of the humerus
• Compression stemming from walking with crutches (called "crutch palsy")
• Pressure from a cast or splint
• Direct trauma, such as an impact or laceration
• Accidental injury during surgery, especially arthroscopic surgery on the inferior glenoid and capsule
• Quadrangular space syndrome, in which the axillary nerve is compressed where it passes through that space (most common in athletes who perform frequent overhead motions)
• Nerve root damage between the fifth and sixth cervical vertebrae, where the nerve emerges from the spinal cord, which can be caused by traction, compression, or spinal disc prolapse ("bulging disc")
• Systemic neurological disorders, such as multiple sclerosis
• Erb's palsy, a condition which often is the result of a birth injury called shoulder dystocia in which a baby's shoulder(s) get stuck during childbirth

Damage may result in axillary nerve palsy, which is a type of peripheral neuropathy (pain from nerve damage) that can cause weakness in the deltoid and teres minor muscles. This can result in the loss of the ability to lift the arm away from the body, as well as weakness in multiple types of shoulder movement.

If the damage is severe enough to cause paralysis of the deltoid and teres minor muscles, it can result in something called "flat shoulder deformity," in which you're unable to lay your shoulder flat when lying down.

Axillary nerve damage also can lead to a change, reduction, or loss of sensation in a small part of the arm just below the shoulder. That area is often referred to as the Sergeant's patch or regimental badge because it's where the stripes would go on the arm of a military uniform.

If your healthcare provider suspects a problem with axillary nerve function, they'll generally test the range of motion in your shoulder and test the sensitivity of the skin. A difference in the range of motion between your shoulders is suggestive of a nerve injury.

To further verify nerve palsy, you may be sent for electromyography and a nerve conduction study. In some cases, an MRI (magnetic resonance imaging) and/or x-rays may be warranted, especially if the cause of possible nerve damage is unknown.

Rehabilitation

Depending on the nature of the injury, non-surgical treatments may be the recommended course, with surgery as a last resort if other treatments aren't sufficient.

Non-surgical treatment for axillary nerve injury may include some combination of immobilization, rest, ice, anti-inflammatory drugs, and physical therapy.

Physical therapy, which typically lasts for about six weeks and focuses on strengthening and stimulated the muscles innervated by the axillary nerve. A major goal is preventing joint stiffness, as that can impair your long-term function.

Surgery

If less invasive treatments fail, surgery may be an option, especially if several months have passed without significant improvement. The outcome is generally better if surgery is performed within six months of the injury, but regardless of the time frame, the prognosis is considered good in about 90% of cases.

Surgical procedures that may be performed for axillary nerve dysfunction or injury include:

• Neurolysis: This involves targeted degeneration of nerve fibers, which interrupts the nerve signals and eliminates pain while the damaged area heals.
• Neurorrhaphy: Basically, this procedure amounts to stitching a severed nerve back together.
• Nerve grafting: Grafting involves transplanting a portion of another nerve, often the sural nerve, to re-connect severed nerves, especially when the damaged portion is too large to be repaired by neurorrhaphy. This allows a pathway for signals and encourages regrowth of nerve axons.
• Neurotization (also called nerve transfer): Similar to grafting but used when the nerve is too damaged to heal, this procedure involves transplanting a healthy but less important nerve, or a portion of it, to replace the damaged nerve and restore function.